This invention relates generally to electromagnetic flowmeters for measuring the flow rate of fluids passing through a pipe line, and more particularly to a flowmeter whose electrode structure is adapted to provide accurate flow measurement in open channels as well as in pipe lines which normally run at less than full capacity.
In an electromagnetic flowmeter, the liquid whose flow rate is to be measured is conducted through a flow tube provided with a pair of diametrically opposed electrodes, a magnetic field normal to the direction of flow being established by an electromagnet. When the flowing liquid intersects this field, a signal is induced therein which is transferred to the electrodes. This signal, which is proportional to the average velocity of the liquid and hence to its average volumetric rate, is then amplified and processed to actuate a recorder or indicator.
Electromagnetic flowmeters of the type heretofore known, including those disclosed in U.S. Pat. Nos. 3,550,446; 3,329,018 and 3,786,687, are effectively restricted to pipe lines which run full. Indeed, the instruction books which accompany such flowmeters usually admonish the user that major inaccuracies would be experienced should the flow tube be interposed in a pipe line that is less than full.
Because of this limitation, existing types of electromagnetic flowmeters are not applicable for measurement in storm sewers, plant effluent lines and in other open-channel and pipe line situations in which less-than-full pipes are normally encountered. The exclusion of electromagnetic flowmeters from such applications is a serious drawback when one takes into account the many advantages to be gained by an electromagnetic flowmeter which is free of moving parts and which introduces no obstacle in the flow path.
There are at least four reasons why known types of electromagnetic flowmeters are incapable of providing accurate measurement with less-than-full pipes, and these will now be briefly considered.